1. Field of the Invention
This invention relates to the field of heat pumps, and more particularly, to a rechargeable thermal energy storage system that utilizes an ice-water reservoir for storing and extracting thermal energy cyclically, by thawing the ice and freezing water intra-seasonally for use in a heat pump system.
2. Description of Related Art
Heat pumps have proven to be extremely useful for lowering energy consumption for home heating in winter and home cooling in summertime. Using a heat pump, the energy required for a heat pump to transfer a given amount of thermal energy, depends directly on the temperature difference between the heat source and receiver from and to which the thermal energy is transferred. This is because a heat pump actually moves heat, rather than creating it. The amount of energy required to move a given volume of heat transfer fluid, is, to the first order, independent of the amount of heat contained in that fluid. So the temperature of the source of heat being pumped into the dwelling being heated in winter, and the temperature of the receiving reservoir for heat that is transferred out of the dwelling being cooled in the summer are crucial for determining the efficiency of a heat pump system for heating or cooling a dwelling.
In the zones of the earth greater than 20xc2x0 latitude from the equator, the rate of input of thermal energy needed to maintain a comfort level of about 20xc2x0 C. in a dwelling is greatest in the fourth and fifth weeks of winter. Obviously, that""s the time of year when the outdoor air is generally at its lowest temperatures. The low thermal mass of a given volume of air, (combined with its low temperature in those periods of winter, makes outdoor air a poor source of thermal energy for home heating whenever the outdoor temperature falls below about 10xc2x0 C. Also, warm outdoor air is a poor receiver of heat for home cooling in summer months when outdoor temperatures rise above about 25xc2x0 C.
If it were practical to store thermal energy in sufficient quantity for six months at a time, thermal energy could be harvested in the warmest summer months, and saved until needed for home heating in the winter months. Conversely, warming the home in the coldest periods of winter, by freezing water and storing the ice produced in the process, could provide a reservoir of a xe2x80x9cheat sinkxe2x80x9d to be used to cool the dwelling over the coming summer. Such a system was proposed and published by C. P. Gilmore, xe2x80x9cHow to Win with ACESxe2x80x9d (Annual Cycling Energy System) in Popular Science, July 1981, pg.49. Gilmore proposed cooling a dwelling by extracting thermal energy from it to melt a large reservoir of ice, and storing the energy in the form of water until wintertime, when it would be refrozen, extracting its latent heat of fusion for heating the dwelling throughout the cold winter months. Several serious problems prevented practical application of the ACES concept. One is that it requires an equal number of heating degree days and cooling degree days, or a balance between the winter heating and summer cooling requirements. A typical home (e.g., a 2-ton house) between about 35xc2x0 to 45xc2x0 latitude requires about one xe2x80x9cton of heatingxe2x80x9d on an average day of mid-winter. One xe2x80x9cton of heatingxe2x80x9d is defined as the rate of thermal energy needed to melt one ton of ice in 24 hrs. That comes to 840 cal/sec. The xe2x80x9ctonsxe2x80x9d heat rating for a given house depends on its size, insulation features, exposure to sun and winds, climate of its location, etc. Bear in mind that the rating states the maximum heat needed on the coldest winter days, which is just about twice the heating rate needed on an average winter day.
Another metric useful for estimating the heating rate required to maintain comfort in a given house on a given day is dependent on the outdoor temperatures, and is expressed in terms of heating degree days. The xe2x80x9cheating degreesxe2x80x9d for a given day is determined by subtracting the average temperature (the sum of the high and low temperatures over 24 hours, divided by two) from 65xc2x0 F. or 18.3xc2x0 C. In geographic latitudes between 35xc2x0 to 45xc2x0, heating degree days per year averages about 5,000 to 8,000. Rochester, N.Y., for example, averages about 6,000 to 6,200 heating degree days Fahrenheit (3,400 htg. deg. days C.) per year.
Since an average temperature on a mid-winter day in those latitudes is about xe2x88x924xc2x0 C., and requires one ton of heating for a 2-ton house, it is found that each ton of heating maintains comfort for a 2-ton house on a 22.3 htg. deg. day C. Thus, one would need 1 ton of heating per 22.3 htg. deg. days C., or 1/22.3 tons=0.045 ton per heating degree day C. For a heating system with a COP of 3.0, based on extracting heat by freezing water, one ton of heating is generated from freezing ⅔ ton, or 0.605 m3 of ice/day. (The other ⅓ ton comes from the electrical power.) One must also conclude, then, that throughout an average season of 6,100 htg. deg. days F. (3,400 htg. Deg. days C.) we need 153 ton-days, which, at ⅔ ton of water to ice per day for each ton of heating, requires freezing 102 tons of water to ice. Each ton of water is 0.907 m3 of water, so 102 tons=92.5 m3 of water, which expands 11% to 102.7 m3 of ice, in volume. That would take a basement of water, about 8 m widexc3x9712 m longxc3x970.96 m deep in water, which, upon freezing would expand upward to 1.07 meters high in ice, if it didn""t expand laterally, crushing the basement walls as it froze.
Thus, the most severe problem of ACES, is to freeze such volumes of water without producing extreme pressures that would deform cooling coils distributed within the water, and would damage the reservoir walls containing the water while freezing it to ice.
The present invention presents solutions to each of these problems by providing a practical means for wintertime heating and summertime cooling of dwellings, while consuming only about ⅓ of the energy required by conventional heating means.
Accordingly, pursuant to the features of the present invention, a FreeTherm thermal energy storage system is disclosed that includes an ice-water reservoir used for storing and extracting thermal energy cyclically, by thawing the ice and freezing water intra-seasonally. A series of heat extractors are positioned within a pool of water that is connected to the reservoir. The heat extractors are adapted to extract heat from water in the process of making ice. Additionally, the heat extractors actively manage the formation of ice to prevent excessive build-up on the heat exchange surfaces that would reduce heat transfer efficiency. The heat extractors are connected to a conventional heat pump and have an anti-freeze fluid that is maintained at sub-freezing temperatures flowing through a bladder attached to a surface thereof. The extractors have concave-downward surfaces for extracting heat from water to produce ice. The extractors are made of flexible, hydrophopic (water repellent) material that can be expanded or bent to release the less flexible ice from their surfaces, and are adapted to pivot up and out of the way to allow the released ice to float to the top of the pool of water. The released ice floats to the surface of the pool of water and is delivered to the reservoir by a conveying mechanism. By maintaining an ice water slurry at the top of the reservoir, potential damage to the reservoir due to freezing water expansion is prevented.